1. Field of the Invention
The present invention relates to a scroll fluid machine, such as a scroll vacuum pump or a scroll compressor, and, more particularly, to a scroll type fluid machine comprising a scroll rotor and a scroll stator and is capable of operating without lubricating the scroll rotor and the scroll stator.
2. Description of the Prior Art
An oil-free scroll fluid machine, in general, comprises a casing; a drive shaft supported for rotation on the casing and provided with an eccentric shaft portion extending within the casing; a scroll rotor consisting of a mirror-finished base plate and a rotor scroll wrap standing upright on the base plate, and supported on a bearing on the eccentric shaft portion of the drive shaft; and a scroll stator attached to the casing, consisting of a base block having a mirror-finished surface and a rotor scroll wrap standing upright on the mirror-finished surface of the base block so as to define a closed space together with the rotor scroll wrap, and provided with a suction port in its peripheral portion and a discharge port in its central portion.
When this oil-free scroll fluid machine is used as a vacuum pump, a closed vessel to be evacuated, such as a vacuum tank, is connected to the suction port, the scroll rotor is revolved by driving the drive shaft via a motor or the like to suck air from the closed vessel into the closed space between the rotor scroll wrap of the scroll rotor and the stator scroll wrap of the scroll stator and to discharge the air through the discharge port. Thus, pumping action is effected to evacuate the closed vessel.
The interior of the casing is in a vacuum state during the pumping action since the suction port is constantly connected to the interior of the casing. Heat generated by the compression of air in the closed space between the scroll rotor and the scroll stator during the pumping action is transmitted through the scroll rotor to the interior of the casing since the casing covers the rear side of the scroll rotor. Further, the radial bearings and the thrust bearing provided in the casing generate frictional heat during the pumping action.
Accordingly, the interior of the casing is heated to a high temperature, thus accelerating the abrasion of the bearings due to insufficient cooling and lubrication of the bearings or, at worst, such an elevated temperature can cause thermal deformation of the scroll rotor.
Although free from problems relating to lubrication and cooling, an oil-cooled vacuum pump or compressor has an undesirable effect in that it discharges air mixed with the lubricant.